This invention relates to hydraulic clamping units of blow molding and injection molding apparata for fabrication of thermoplastic articles.
In the injection molding method, a heat-softened plastic material is forced from a cylinder into a mold cavity under hydraulic pressure of the plastic.
In the blow molding method, a warm plastic parison (hollow tube) is formed either by injection molding (injection blow molding process), or by an extruder (extrusion blow molding process), is placed between two halves of a mold and is assumed the shape of a cavity of the mold under air pressure inside the parison.
Known hydraulic clamping units (for example, U.S. Pat. No. 3,078,508; Society of the Plastics Industry. Plastics Engineering Handbook of the Society of Plastics Industry, Inc. New York, Van Nostrand Reinhold Co., 1976, pp. 87-90, 102-103; Ed Galli. Drumming up New Business in Large Blowmolded Containers. Plastics Machinery and Equipment, October, 1986, p. 22) include a pair of opposed complimentary mold halves attached accordingly to a pair of opposed platens, at least one of which is moveable toward another into engagement of the mold halves to form a mold cavity which surrounds a molded article of a thermoplastic material, said moveable platen being connected to one of two ends of at least one hydraulic cylinder exerting pressure on the platens to keep the mold halves closed during formation of the article.
Herein the hydraulic clamping unit means a full hydraulic unit actuated by a hydraulic cylinder which is directly connected to the moving platen. This is in contrast to a hydromechanic unit which also has a hydraulic cylinder, but the latter is connected to the platen not directly, but by means of a toggle mechanism.
One disadvantage of the known units lies in the use of a push-type clamp cylinders attached between the moving platen and a frame member. This arrangement requires relatively large diameters of the piston rods of the cylinders and large frame members to withstand high hydraulic pressure.
In blow molding and large injection molding apparata, another drawback is large hydraulic capacities. This is overcome in so-called lock-and-block units (see the above-cited handbook, pp. 102-103) wherein a small-diameter high-speed rapid traverse kicker cylinder is used in addition to the main clamp cylinder. At the end of a stroke of the kicker cylinder, a hydromechanical locking mechanism is employed to lock the main clamp cylinder. In these units, the kicker cylinder and the main cylinder are installed in-line, with high loads transmitted to all other components of the units. Also, the kicker cylinder therein still requires a relatively high capacity of the hydraulic system.
Further drawback of the known clamping units with kicker cylinders lies in having only one platen moving and thus causing vibration. It should be mentioned that units with both platens moving are known (Michael Yahr. Extrusion-blow Molding. Modern Plastics Encyclopedia, 1983-84, p. 179). However, this unit does not have kicker cylinders.
Finally, most known units have platens sliding in tie rods, which results in extensive wear.